GRAIN-REFINEMENT MECHANISM IN UNDERCOOLED CU30NI70

When undercooled molten Cu30Ni70 crystallizes at an undercooling Delta T congruent to 145 K (Delta T is defined as T-l - T-k where T-l is th e liquidus of Cu30Ni70 and T-k is the kinetic crystallization temperat ure), its grain size undergoes a rapid decrease by as much as two orde rs of magnitude in a narrow temperature range, This phenomenon is term ed grain refinement. It was found that grain refinement is brought abo ut by multiplication of dendrites. Composition analysis of the dendrit e indicates that it has the least Ni concentration at its axis. The Ni content then increases radially from the central axis. Therefore, the dendrite is unstable against melting since the melting temperature of Cu-Ni increases with Ni content. The origin of grain refinement is at tributed to the remelting of these dendrites.